Editing Adhesion (section)

== Surface energy ==
[[File:Lease044.jpg|thumb|Diagram of various cases of cleavage, with each unique species labeled.<br> '''A''': γ = (1/2)W<sub>11</sub><br>
'''B''': W<sub>12</sub> = γ<sub>1</sub> + γ<sub>2</sub> – γ<sub>12</sub><br>
'''C''': γ<sub>12</sub> = (1/2)W<sub>121</sub> = (1/2)W<sub>212</sub><br>
'''D''': W<sub>12</sub> + W<sub>33</sub> – W<sub>13</sub> – W<sub>23</sub> = W<sub>132</sub>.]]

[[Surface energy]] is conventionally defined as the [[work (physics)|work]] that is required to build an area of a particular [[surface]]. Another way to view the surface energy is to relate it to the work required to cleave a bulk sample, creating two surfaces. If the new surfaces are identical, the surface energy γ of each surface is equal to half the work of cleavage, W: γ = (1/2)W<sub>11</sub>.

If the surfaces are unequal, the [[Young-Dupre equation|Young-Dupré equation]] applies:
W<sub>12</sub> = γ<sub>1</sub> + γ<sub>2</sub> – γ<sub>12</sub>, where γ<sub>1</sub> and γ<sub>2</sub> are the surface energies of the two new surfaces, and γ<sub>12</sub> is the interfacial energy.

This methodology can also be used to discuss [[Bond cleavage|cleavage]] that happens in another medium: γ<sub>12</sub> = (1/2)W<sub>121</sub> = (1/2)W<sub>212</sub>. These two energy quantities refer to the energy that is needed to cleave one species into two pieces while it is contained in a medium of the other species. Likewise for a three species system: γ<sub>13</sub> + γ<sub>23</sub> – γ<sub>12</sub> = W<sub>12</sub> + W<sub>33</sub> – W<sub>13</sub> – W<sub>23</sub> = W<sub>132</sub>, where W<sub>132</sub> is the energy of cleaving species 1 from species 2 in a medium of species 3.<ref name=Israelachveli/>

A basic understanding of the terminology of [[cleavage energy]], surface energy, and [[surface tension]] is very helpful for understanding the [[physical state]] and the events that happen at a given surface, but as discussed below, the theory of these variables also yields some interesting effects that concern the practicality of adhesive surfaces in relation to their surroundings.<ref name=Israelachveli>J. N. Israelachvili, ''Intermolecular and Surface Forces'' (Academic Press, New York, 1985). chap. 15.</ref>

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